It is known in the art to manually or automatically control screening devices related to windows, e.g. curtains, roller shutters, blinds, sunshades, awnings or the like to avoid the sun to dazzle persons or to overcome that the inside temperature is influenced too much by the outside temperature or by sun heating. The screening device may be manually controlled, typically by means of a person turning a handle or pushing a button on a remote control which activates a motorized actuator related to the screening device. In some relatively new systems, the screening device is self controlling which means that the screening device acts upon receiving input from one or several sensors which e.g. measures light intensity or temperature. The screening device may then e.g. screen the window if the interior temperature gets to high. An example of a system which controls a screening device by means of different sensors is presented in U.S. Pat. No. 6,064,949 which discloses a method of controlling a screening device based on a number of factors, e.g. climate conditions.
Another example of an automatic control of a screening device is disclosed in international patent application WO 2007/006775. Here, a blind is controlled inter alia by determining the weather conditions by means of a photo sensor and by calculating the position of the sun by means of determining a geodetic position with a GPS receiver.
However, several problems may arise in sensor dependent system like the abovementioned. In a system with several sensors situations may occur where two independent sensors may send inconsistent instructions to a screening device. Or one or several sensors may subsequently send opposing instructions to a screening device of which e.g. will result in an unintended and undesirable constant opening and closing of a screening device. If the system has reduced electric power, e.g. battery powered or solar cell powered, then power consumption is high when sensors and controller communicate, e.g. pull or send information. It is desirable to make an automation system for energy optimization which is simple and cost efficient.